Health-Promoting School (HPS) interventions aim to reduce bullying and violence via curriculum, environmental and family/community-engagement components. Despite evidence of their effectiveness, factors influencing the implementation of such interventions are poorly understood. This systematic review aims to examine such factors by assessing qualitative process evaluations of HPS interventions aiming to reduce bullying, aggression or violence. A comprehensive systematic search of 12 databases was carried out, and 20 reports from 17 studies were included. Thematic synthesis was used to identify factors affecting implementation. Factors that enable implementation were related to programme characteristics and stakeholder buy-in, including support from leadership, teachers, students and parents. Good communication and staff climate were important. Interventions were better implemented when they framed health promotion as a core school business, were supported by a national policy, used local data to show need and effectiveness and provided high-quality, pragmatic and accessible staff training. The results of this review can serve to guide and facilitate the design and implementation of future bullying and violence prevention programmes. Since there is significant overlap in terms of the important pillars and guiding principles for all interventions guided by the HPS framework, the findings may apply to outcomes beyond bullying and violence.